Docklevelers are mounted on loading docks and are adapted to bridge the gap between the dock and the end of a vehicle (e.g., a truck or trailer) parked in front of the dock. The dockleveler enables material handling equipment, such as a fork lift, to move between the dock and the vehicle.
The typical dockleveler includes a support structure, which is mounted in a pit in the loading dock, and a ramp. The rear end of the ramp is pivotally attached to the support structure so that the ramp is movable from a horizontal position to a position within the working range of the ramp (e.g., between an upwardly inclined position and a downwardly declined position). Hinged to the front end of most ramps is an extension lip, which is movable between a downwardly hanging pendent position and an extended position where the lip forms an extension to the ramp. After a truck is parked in front of the loading dock in preparation for a loading operation, the ramp is pivoted upwardly to allow the pendent lip to clear the vehicle bed. As the ramp is moved, the lip is pivoted outwardly from the pendent position to the extended position. As the ramp is lowered, the extended lip will engage the bed of the truck to provide support to the ramp.
Many docklevelers include a biasing mechanism for upwardly-biasing the ramp. For example, some docklevelers include a large spring that biases the ramp toward the inclined position. The biasing force is such that it is only slightly more than is enough to counterbalance the weight of the ramp. In order to lower the ramp into engagement with a vehicle, the user can walk onto the ramp, thereby overcoming the biasing force, and lowering until the lip engages the vehicle.
In order to maintain the lip in contact with the vehicle, some docklevelers are provided with a holdown mechanism connected between the ramp and the support structure. One type of holdown mechanism includes a ratchet bar connected to the ramp and a spring assembly connected to the support structure. The spring assembly includes a compression spring positioned within a hollow cylinder, and a pawl device connected to the cylinder. The pawl device slidably receives the ratchet bar and can selectively engage the ratchet bar to prevent it from moving upwardly away from the pawl device. In this engaged condition, the compression spring will allow some upward movement of the ratchet bar relative to the support structure to thereby accommodate upward float of the vehicle. Such a device is disclosed in U.S. Pat. No. 3,646,627.
One type of holdown mechanism has been designed to accommodate a greater amount of upward float. This design includes a release cable that will release engagement between the pawl and ratchet bar when the compression limit of the compression spring is approached. This design is disclosed in U.S. Pat. No. 4,531,248.
An alternative means for increasing the amount of float that can be accommodated by a holdown mechanism is to increase the length of the compression spring. However, due to space constraints under many ramps (e.g., when the ramp is in the declined position), the extent to which the compression spring can be lengthened is limited.